1. Field of the Invention
The present invention relates to a semiconductor laser for generating a laser beam and for receiving the laser beam reflected by an object, and to an optical pickup device using the semiconductor laser device.
2. Description of the Prior Art
In optical recording apparatuses using CD-ROMs, MDs (MiniDisk™) or other optical discs as recording media, optical pickups are used for reading signals from the optical disc. Some optical pickups use semiconductor laser devices of hologram laser type. An example of such semiconductor laser devices is disclosed in the Japanese Unexamined Patent Publication No. H06-203403. In this device, a semiconductor laser element, a holographic element and a photo-detecting element are built in a package. The semiconductor laser element generates a laser beam, and the holographic element diffracts the laser beam reflected by an optical disc so that the reflected beam is introduced into the photo-detecting element located apart from the semiconductor laser element.
In the semiconductor laser device disclosed in the aforementioned publication, a mounting part on which the semiconductor laser element is placed is formed in a semiconductor chip used as the photo-detecting element, and the semiconductor laser element is mounted on the photo-detecting element so that both elements constitute a unit. Therefore, when the relative position of the semiconductor laser element and the photo-detecting element should be changed due to, for example, some modification in specification, the position and/or height of the mounting section should be accordingly modified, so that the photo-detecting element itself needs to be newly produced. This is not desirable with respect to the production efficiency.
Accordingly, the applicant has proposed a construction wherein the semiconductor laser element is separated from the photo-receiving element, and both elements are mounted on different mounting bases, as disclosed in the Japanese Unexamined Patent Publication No. 2003-264332 (Japanese Patent Application No. 2002-066601). By this construction, the relative position of the semiconductor laser element and the photo-detecting element can be changed by newly producing only the mounting base having a simple structure, as opposed to the photo-detecting element having a complex structure.
FIGS. 6A–6D show the construction of this semiconductor laser device, where FIG. 6A is a plan view, FIG. 6B is a side view, FIG. 6C is a perspective view and FIG. 6D is another perspective view in which the holographic element is removed to show the inside of the device. In these drawings, numeral 1 denotes a semiconductor laser element, 2 denotes a reflecting mirror, 10 denotes a lead frame, 4 denotes a sub-mount with a monitoring photodiode, 5 denotes a photo-detecting element for reading signals (which consists of a photodiode integrated with a signal-processing circuit), and 6 denotes a holographic element. The holographic element 6 consists of a transparent member (made of glass or resin) having a grating pattern for generating a three-beam formed on its lower face and a holographic pattern for splitting the beam formed on its upper face.
In the construction of FIGS. 6A–6D, the package of the device consists of a body frame 60 made of an insulating resin with a base plate 60a and a sidewall 60b extending along the edge of the base plate 60a. The sidewall 60b has a pair of arc sections 60c facing each other and a pair of straight sections 60d facing each other. The holographic element 6 is mounted on the straight sections 60d. The lead frame 10 extending to the outside, the semiconductor laser element 1, the photo-detecting element 5 and the mirror 2 are located inside the body frame 60. The semiconductor laser element 1 is mounted on the sub-mount 4, which functions as the mounting base. The photo-detecting element 5 and the mirror 2 are each mounted on a separate mounting base made of a resin. The electrodes of the semiconductor laser element 1 and the photo-detecting element 5 are electrically connected to the lead frame 10 by Au (gold) wires 8.
The mirror 2 is located in front of the semiconductor laser element 1 and reflects the laser beam generated by the semiconductor laser element 1 so that the beam changes its direction by 90 degrees and enters the holographic element 6. The photo-detecting element 5 is located one a side of mirror 2 and receives the laser beam reflected by an optical disc and then diffracted by the holographic element 6 into a different direction.
FIGS. 7A–7C show the inside of the above-described semiconductor laser device, where FIG. 7A is the sectional view at line A—A in FIG. 6A, FIG. 7B is the sectional view at line B—B, and FIG. 7C is the sectional view at line C—C. FIGS. 7A and 7B correspond to Y-Z sections on the Cartesian coordinates shown in FIG. 6C. Similarly, FIG. 7C corresponds to an X-Z section.
The detector-mounting base 32 for the photo-detecting element 5 is connected to the straight section 60d of the sidewall 60b of the body frame 60 and located on the island plate 30 of the lead frame 10. Similarly, the mirror-mounting base 33 for the mirror 2 is connected to the straight section 60d of the sidewall 60b of the body frame 60 and located on the island plate 30 of the lead frame 10. The detector-mounting base 32 and the mirror-mounting base 33 are combined together.
In operation, semiconductor laser devices generate heat and have high temperatures. Also, they are often used under the conditions of high temperature and high humidity. As a result, the body frame and the mounting bases, both made of resin, expand due to a rise in temperature or moisture absorption. By the above-described construction, expansion of the body frame 60 causes the detector-mounting base 32 and the mirror-mounting base 33, both connected to the body frame 60, to change their positions. In particular, with reference to the aforementioned coordinates, the expansion of the body frame 60 causes the detector-mounting base 32 and the mirror-mounting base 33, both connected to the body frame 60, to move in the negative direction of the Y-axis. The expansion of the detector-mounting base 32 and the mirror-mounting base 33, which are combined together, cause them to move away from each other; the detector-mounting base 32 moves in the positive direction of the X-axis, and the mirror-mounting base 33 moves in the negative direction of the X-axis. In this case, either of the two bases that is less resistant to external forces will be displaced.
Displacement of these mounting bases 32, 33 would lead to an error in the optical configuration of the elements 1, 2, 5 and 6, which are normally adjusted to the optimal positions. Consequently, the convergence position of the laser beam on the photo-detecting element 5 would change, so that the photo-detecting element 5 could not receive the laser beam in the optimal condition. Such a condition of the photo-detecting element 5 would deteriorate the reliability of the optical pickup device in reading signals.